Characterization of n-alkanes, pristane and phytane in the Cretaceous/Tertiary boundary sediments at Kawaruppu, Hokkaido, Japan

Abstract

Normal alkanes from C₁₂ to C₃₆, pristane and phytane were identified in the Cretaceous/Tertiary (K/T) boundary sediments at Kawaruppu, Hokkaido, Japan. These compounds were found at the levels of sub- to a few nmol g^-1. Total concentrations of the n-alkanes within the boundary claystone were generally one third to one half of those in the sediments above and below the claystone. The smaller concentrations within the claystone were mainly due to smaller concentrations of longer chain n-alkanes (C₂₅ to C₃₁) than in the sediments above and below it. The longer chain n-alkanes were likely from terrestrial plants and reflected their small population at the end of Cretaceous. The concentration of longer chain n-alkanes decreased rapidly at the base of the boundary claystone, indicating the sudden cease of terrestrial n-alkane input, continued low to the horizon at the upper two thirds of the claystone and then, started increasing toward the top of the claystone. The n-alkane concentration change indicates the period of the small input of terrestrial n-alkanes to be ca. 7, 000 years at most and the recovery period to be ca. 2, 000 years. The depth distribution pattern of pristane plus phytane concentrations in the K/T sediments roughly resembles that of the n-alkanes.